[tracing] Send unique tracing process id for cross-process memory dumps identification.

base/trace_event/memory_allocator_dump_guid.cc

Index: base/trace_event/memory_allocator_dump_guid.cc
diff --git a/base/trace_event/memory_allocator_dump_guid.cc b/base/trace_event/memory_allocator_dump_guid.cc
index a4ea50d416a0d5bd784e7c74501f77ad45d7b49f..9c770bfa66a212852c8d117c18d0d675fc90979b 100644
--- a/base/trace_event/memory_allocator_dump_guid.cc
+++ b/base/trace_event/memory_allocator_dump_guid.cc
@@ -6,11 +6,28 @@
 
 #include "base/format_macros.h"
 #include "base/hash.h"
+#include "base/strings/string_number_conversions.h"
 #include "base/strings/stringprintf.h"
 
 namespace base {
 namespace trace_event {
 
+namespace {
+int g_unique_child_process_id = MemoryAllocatorDumpGuid::kInvalidChildProcessId;
+
+uint64 ExpandChildProcessIdAndHash(std::string guid_str) {

[dcheng, 2015/06/20 00:36:27]

const std::string&

[Primiano Tucci (use gerrit), 2015/06/20 12:32:31]

Note: the string is mangled below. If he passes a const& string, than he'll need
to create a copy anyways to mangle it. is it worth it the extra line?

[ssid, 2015/06/20 13:07:24]

Yes, instead of creating a copy, i passed std::string.

+  const char kReplaceChildIdStr[] = "$$";
+  size_t pos = guid_str.find(kReplaceChildIdStr);
+  if (pos != std::string::npos) {
+    DCHECK_NE(g_unique_child_process_id,
+              MemoryAllocatorDumpGuid::kInvalidChildProcessId);
+    guid_str.replace(pos, sizeof(kReplaceChildIdStr) - 1,
+                     IntToString(g_unique_child_process_id));
+  }
+  return Hash(guid_str);
+}
+}  // namespace
+
 MemoryAllocatorDumpGuid::MemoryAllocatorDumpGuid(uint64 guid) : guid_(guid) {
 }
 
@@ -19,7 +36,14 @@ MemoryAllocatorDumpGuid::MemoryAllocatorDumpGuid()
 }
 
 MemoryAllocatorDumpGuid::MemoryAllocatorDumpGuid(const std::string& guid_str)
-    : MemoryAllocatorDumpGuid(Hash(guid_str)) {
+    : MemoryAllocatorDumpGuid(ExpandChildProcessIdAndHash(guid_str)) {
+}
+
+// static
+void MemoryAllocatorDumpGuid::SetUniqueChildProcessId(int child_process_id) {
+  DCHECK(g_unique_child_process_id == kInvalidChildProcessId ||
+         child_process_id == kInvalidChildProcessId);
+  g_unique_child_process_id = child_process_id;
 }
 
 std::string MemoryAllocatorDumpGuid::ToString() const {